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Full text of "Slavery in ants."

SLAVERY IN ANTS 



Certain species of ants raid the nests of other species f< 



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k in their own nest. Some raiding species have hecome so 



specialized that they are no longer capable of feeding tl 



g rnemseives 



I 



by Edward O. Wilson 



The institution of slavery is not 
unique to human societies. No 
fewer than 35 species of ants, 
constituting six independently evolved 
groups, depend at least to some extent 
on slave labor for their existence. The 
techniques by which they raid other ant 
colonies to strengthen their labor force 
rank among the most sophisticated be- 
havior patterns found anywhere in the 
3ct world. Most of the slave-making 
ant species are so specialized as raiders 
that they starve to death if they are de- 
prived of their slaves. Together they dis- 
play an evolutionary descent that begins 
with casual raiding by otherwise free- 
living colonies, passes through the de- 
velopment of full-blown warrior societies 
and ends with a degeneration so ad- 
vanced that the workers can no longer 
even conduct raids. 

Slavery in ants differs from slavery in 
human societies in one key respect: the 
ant slaves are always members of other 
completely free-living species that them- 
selves do not take slaves. In this regard 
the ant slaves perhaps more closely re- 
semble domestic animals except that 
the slaves are not allowed to reproduce 
and they are equal or superior to their 
captors in social organization. 



r l^\\e famous Amazon ants of the genus 
Pohjergus are excellent examples of 
advanced slave makers. The workers are 
strongly specialized for fighting. Their 
mandibles, which are shaped like minia- 
ture sabers, are ideally suited for punc- 
turing the bodies of other ants but are 
poorly suited for any of the routine tasks 
that occupy ordinary ant workers. In- 
deed, when Pohjergus ants are in their 
home nest their only activities are beg- 
ging food from their slaves and cleaning 
themselves ("burnishing their ruddy ar- 
mor," as the entomologist William Mor- 
ton Wheeler once put it). 

32 



When Pohjergus ants launch a raid, 
however, they are completely trans- 
formed. They swarm out of the nest in 
a -solid phalanx and march swiftly and 
directly to a nest of the slave species. 
They destroy the resisting defenders by 
puncturing their bodies and then seize 
and carry off the cocoons containing tl 
pupae of worker ants. 

When the captured pupae hutch, the 
workers that emerge accept their cap- 
tors as sisters; they make no distinction 
between their genetic siblings and the 



K' 



Pohjergus ants. The workers launch into 
the round of tasks for which they have 
been genetically programmed, with the 
slave makers being the incidental bene- 
ficiaries. Since the slaves are members of 
the worker caste, they cannot reproduce. 
In order to maintain an adequate labor 
force, the slave-making ants must peri- 
odically conduct additional raids. 

It is a remarkable facH that ants of 
slave-making species are found only in 
cold climates. Although the vast major- 
ity of ants live in the Tropics and the 



* 







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RAID BY SLAVE-MAKING AMA/ON ANTS of the species Polyergus rufescens (light 

color) against a colony of the slave species Formica fusca (dark color) is depicted. The 
fusca ants make their nest in dry soil under a stone. The raiding Amazon ants kill resisting 



warm Temperate zones, not a single spe- 
cies of those regions has been implicat- 
ed in any activity remotely approaching 
slavery. Among the ants of the colder 
regions this form of parasitism is surpris- 
ingly common. The colonies of many 
slave-making species abound in the for- 
ests of the northern U.S., and ant-slave 
raids can be observed in such unlikely 
places as the campus of Harvard Uni- 
versity. 

The slave raiders obey what is of- 
ten called Emery's rule. In 1909 Carlo 
Emery, an Italian myrmecologist, noted 
that each species of parasitic ant is ge- 
netically relatively close to the species it 
victimizes. This .relation can be profit- 
, ably explored for the clues it provides to 
the origin of slave making in the evolu- 
tion of ants. Charles Darwin, who was 
fascinated by ant slavery, suggested that 
the first step was simple predation: the 
itral species began by raiding other 
kinds of ants for food, carrying away 
their immature forms in order to be able 
to devour them in the home nest. If a 
few pupae could escape that fate long 
enough to emerge as workers, they might 
be accepted as nestmatcs and thus join 
the labor force. In cases where the cap- 
tives subsequently proved to be more 
valuable as workers than as food, the 



raiding species would tend to evolve 

into a slave maker. 

Although Darwin's hypothesis is at- 
tractive, I recently obtained evidence 
that territorial defense rather than food 
is the evolutionary prime mover. I 
brought together in the Harvard Mu- 
seum of Comparative Zoology different 
species of LeptotJtorax ants that normal- 
ly do not depend on slave labor. When 
colonies were placed closer together 
than they are found in nature, the larger 
colonies attacked the smaller ones and 
drove away or killed the queens and 
workers. The attackers carried captured 
pupae back to their own nests. The pu- 
pae were then allowed by their captors 
to develop into workers. In the cases 
where the newly emerged workers be- 
longed to the same species, they were 
allowed to remain as active members of 
the colony. When they belonged to a 
different Leptothorax species, however, 
they were executed in a matter of hours. 
One can easily imagine the origin of 
slave making by the simple extension of 
this territorial behavior to include toler- 
ance of the workers of related species. 
The more closely related the raiders and 
their captives are, the more likely they 
are to be compatible. The result would 
be in agreement with Emery's rule. 




One species that appears to have just 
crossed the threshold to slave making is 
Leptothorax. duloticus, a rare ant that 
so far has been found only in certain 
localities in Ohio, Michigan and Ontario. 
The anatomy of the worker caste is only 
slightly modified for slave-making be- 
havior, suggesting that in evolutionary 
terms the species may have taken up its 
parasitic way of life rather recently. 

n experiments with laboratory colonies 
I was able to measure the degree of 
behavioral degeneration that has taken 
place in L. duloticus. Like the Amazon 
ants, the duloticus workers are highly 
efficient at raiding and fighting. When 
colonies of other Leptothorax species 
were placed near a duloticus nest, the 
workers launched intense attacks until 
all the pupae of the other species had 
been captured. 

In the home nest the duloticus work- 
ers were inactive, leaving almost all the 
ordinary work to their captives. When 
tin- sl;i\r.s \vcn- temporarily taken away 
from them, the workers displayed a dra- 
matic expansion in activity, rapidly tak- 
ing over most of the tasks formerly car- 
ried out by the slaves. The duloticus 
workers thus retain a latent capacity for 
working, a capacity that is totally lack- 



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fitsca workers by piercing them with their saberlike mandibles. 
Most of the Amazon ants are transporting cocoons containing tbe 
pupae of fusca workers back to their own nest. When the workers 



*rge from the cocoons, they serve as slaves. Two dead fusca 
workers that resisted lie on the ground. Two other workers have 
retreated to upper surface of the rock over the nest's entrance. 



33 



ing in more advanced species of slave- 
making ants. 

The duloticus workers that had lost 
their slaves did not, however, perform 
their tasks well. Their larvae were fed 
at infrequent intervals and were not 
groomed properly, nest materials were 
ied about aimlessly and were never 
placed in the correct positions, and an 
inordinate amount of time was spent 
collecting and sharing diluted honey. 
More important, the slaveless ants lacked 
one behavior pattern that is essential for 



the survival of the colony: foraging for 
dead insects and other solid food. They 
even ignored food placed in their path. 
When the colony began to display signs 
of starvation and deterioration, I re- 
turned to them some slaves of the spe- 
cies Leptolhorax curvispinosus. The 
bustling slave workers soon put the nest 
back in good order, and the slave makers 
just as quickly lapsed into their usual 
indolent ways. 

Not all slave-making ants depend on 
brute force to overpower their victims. 



Quite by accident Fred E. Rcgnier of 
Purdue University and I discovered that 
some species have a subtler strategy. 
While surveying chemical substances 



^ 



used by ants to communicate alarm and 
to defend their nest, we encountered two 
slave-making species whose substances 
differ drastically from those of all other 
ants examined so far. -These ants, For- 
mica subintcgra and Formica pcrgandei, 
produce remarkably large quantities of 
decyl, dodecyl and tetradecyl acetates. 
Further investigation of F. aubintcgra 



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. -** * * 



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INTERIOR VIEW OF THE HOME NEST of a colony of Amazon 
ants shows formica fusca slaves (dark color) performing all the 
housekeeping labor. At top center one of the slaves brings a fly 
wing into the nest for food. Other slave workers care for the small 
eggs, grublike larvae and cocoon-enclosed pupae of their captors. 



During the raiding season some of the pupae are likely to he those 
of fusca workers. The slave makers (light color) can do nothing 
more than groom themselves (upper left). In order to cat, the 
Amazon ants niuM beg slave workers to regurgitate liquid drop- 
lets for llicm llttiver left). These ant species are found in Europe. 




34 



revealed that the substances are sprayed 
at resisting ants during slave-making 
raids. The acetates attract more invading 
slave makers, thereby serving to assem- 
ble these ants in places where fighting 
breaks out. Simultaneously the sprayed 
acetates throw the resisting ants into a 
panic. Indeed, the acetates are excep- 
tionally powerful and persistent alarm 
substances. They imitate the compound 
undecane and other scents found in slave 
species of Formica, which release these 
substances in order to alert their nest- 
mates to danger. The acetates broadcast 
by the slave makers are so much strong- 
er, however, that they have a long-last- 
ing disruptive effect. For this reason 
Kegnier and I named them "propaganda 
substances." 

We believe we have explained an odd 
fact first noted by Pierre Huber 165 
years ago in his pioneering study of the 
European slave-making ant Formica 
san guinea. He found that when a colony 
was attacked by these slave makers, the 
survivors of the attacked colony were 
reluctant to stay in the same neighbor- 
hood even when suitable alternative nest 
sites were scarce. Huber observed that 
the "ants never return to their besieged 

^^ 

capital, even when the oppressors have 
retired to their own garrison; perhaps 
they realize that they could never re- 
main there in safety, being continually 
liable to the attacks of their unwelcome 



DUFOUR'S GLAND 






vstors. 

Regnier and I were further able to 
gain a strong clue to the initial organi- 
zation of slave-making raids. We had 
made a guess, based on knowledge of the 
foraging techniques of other kinds of 
ants, that scout workt-rs direct their nest- 
mates to newly discovered slave colonies 
by means of odor trails laid from the tar- 
get back to the home nest. In order to 
test this hypothesis we made extracts of 
the bodies of F. subintegra and of For- 
mica rubicitnda, a second species that 
conducts frequent, well-organized raids 
through much of the summer. Then at 
the time of day when raids are normally 
made we laid artificial odor trails, using 
a narrow paintbrush dipped in the ex- 
tracts we had obtained from the ants. 
The trails were traced from the entrances 
of the nest to arbitrarily selected points 
one or two meters away. 

The results wore dramatic. Many of 

J 

the slave-making workers rushed forth, 
ran the length of the trails and then 
milled around in confusion at the end. 
When we placed portions of colonies of 
the slave species Formica subscricea at 
the end of some of the trails, the slave 
makers proceeded to conduct the raid in 
a manner that was apparently the same 



POISON GLAND 



MID-INTESTINE 



RECTAL SAC 




DUFOUR'S GLAND 



DUFOUR'S GLAND, which produces substances that nerve as communication scents among 
ants, is much larger in the slave-making species Formica subintegra (top) limn in the slave 
species F. sitbsericea (bottom). The stibscricea ant releases its scent to alert its nestmates 
to the presence of danger. The subintegra sprays its secretions at resisting ants during 
slave raids. The secretions are so strong that they create panic in the colony bring attacked. 



in every respect as the raids initiated by 
trails laid by their own scouts. Studies of 
the slave-making species Pohjergus lu- 
ci(his and Harpagoxcnus amcricanus by 
Mary Talbot and her colleagues at Lin- 
denwood College provide independent 
evidence that raids are organized by the 
laying of odor trails to target nests; in- 
deed, this form of communication may 
be widespread among slave-making ants. 

r Phe evolution of social parasitism in 
ants works like a ratchet, allowing a 
species to slip further down in parasitic 
dependence but not back up toward its 
original free-living existence. An exam- 
ple of nearly complete behavioral de- 
generation is found in one species of the 
genus Strongylognatlms, which is found 
in Asia and Europe. Most species in this 
genus conduct aggressive slave-making 
raids. They have characteristic saber- 



shaped mandibles for killing other ants. 
The species StrongylognatJnts testaceus, 
however, has lost its warrior habits. Al- 
though these ants still have the distinc- 

O 

tive mandibles of their genus, they do 
not conduct slave-making raids. Instead 
an S. testaceus queen moves into the 
nest of a slave-ant species and lives 
alongside the queen of the slave species. 
Each queen lays eggs that develop into 
workers, but the S. testaceus offspring 
do no work. They are fed by workers of 
the slave species. We do not know how 
the union of the two queens is formed in 
the first place, but it is likely that the 
parasitic queen simply induces the host 
colony to adopt her after her solitary dis- 
persal flight from the nest of her birth. 

Thus S. testaceus is no longer a real 
slave maker. It has become an advanced 
social parasite of a kind that commonly 
infests other ant groups. For example, 



35 



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RESEMBLANCE of slave maker and slave was noted by an Italian 
myrmecologist, Carlo Emery, in 1909. In each pair of ants shown 
here the slave maker is on the left and the slave on the right. The 
species depicted are (a) Polyergus rufescens and Formica fusca, 



r 







i 

(6) Rossomyrmex proformicarum and Proformica nasutum, (c) 
Harpagoxenus americanus and Leptothorax curvispinosus, (d) L, 
(hiloticus and L. CUrvispinotUS, (e) Strongylognnthus alpinus and 
Tetramorium caespitum and (/) F. subintegra and F. subsericea. 



many species of ant play host to para- 
sites such as beetles, wasps and flies, 
feeding them and sheltering them [see 
"Communication between Ants and 
Their Guests," by Bert Holldobler; SCI- 
ENTIFIC AMERICAN, March, 1971]. 

Does ant slavery hold any lesson for 



our own species? Probably not. Human 
slavery is an unstable social institution 
that runs strongly counter to the moral 
systems of the great majority of human 
societies. Ant slavery is a genetic adapta- 
tion found in particular species that can- 
not be judged to be more or less success- 



ful than their non-slave-making counter- 
parts. The slave-making ants offer a clear 
and interesting case of behavioral evolu- 
tion, but the analogies with human be- 
havior are much too remote to allow us 
to find in them any moral or political 
lesson. 










COLONY OF ANTS housed in a glass tube consists of the rare 
species Leptothornx duloticus and a slave species, L. curvispinosus, 
The duloticus ant, found in Ohio, Michigan and Ontario, has only 
recently become a slave maker. One of the duloticus workers can 

36 



be seen in the center of the photograph; below it are three slave 
workers. The white objects are immature forms of both species. 
When the slave workers are removed, the duloticus workers at- 
tempt to carry out necessary housekeeping tasks but do so poorly.